Many types of small-wheeled, self-propelled devices are burdened by limitations caused by the ongoing tension between optimal performance and economic cost. Too often performance is sacrificed to lower costs, and the result is an inferior product. Taking the opposite approach and sacrificing low cost for optimal performance, however, often leads to poor sales because customers are not willing to accept disproportionately higher prices. A better situation, therefore, is one in which performance is increased with only a slight cost increase. In this situation, customers are more willing to accept a marginally higher price to buy a better product. Still better situations arise when performance increases and costs remain the same or decrease.
An example of small-wheeled devices where customers routinely consider costs and benefits is walk-behind power equipment. Many forms of this equipment that were once solely push-propelled more and more are becoming self- propelled. Examples of such equipment are lawnmowers, lawn vacuums, snow throwers, flexible line trimmers, and the like. A number of factors have driven the move to self-propulsion, such as a desire for larger equipment with less effort required to use the equipment. Current self-propulsion systems for walk-behind equipment generally fall into two categories, simple and complex. Each category has limitations and disadvantages.
One type of simple drive system comprises a belt-and-sheave power take-off that drives a pair of wheels. In this system, a motor engages a belt, which extends around a sheave or pulley on an axle attached to the wheels. The motor drives the belt around the pulley producing rotation of the axle. The user engages and disengages this type of drive system by adjusting the tension on the drive belt. Typically, such a drive system is either fully engaged (a taught belt) or fully disengaged (a slack belt). A chief disadvantage of a belt-driven system is that the common axle drives the two connected wheels at the same rate. Because differential rotation between the drive wheels is not permitted, cornering with a piece of equipment having a belt and sheave drive system can be difficult.
More particularly, cornering requires the outer wheel to travel a greater distance than the inner wheel. Because the outer wheel must travel farther than the inner wheel in the same amount of time, the outer wheel must rotate faster than the inner wheel. When the outer and inner wheels are fixed to a common axle, however, this differential rotation is not permitted. The result is that either the inner wheel is driven faster or the outer wheel is driven slower than necessary. In either case, cornering the equipment requires one of the wheels to slip or skid.
Difficulty with cornering and wheel slippage are two major disadvantages with using equipment having drive wheels fixed to a common axle. Additionally, effort by the operator must be provided to overcome the ground-engaging forces to allow one wheel to slip. Furthermore, wheel slippage can cause damage to the surfaces on which the equipment is operating, as well as accelerated tire wear. For instance, turning a lawnmower with this type of drive system damages the turf under the slipping wheel.
Power equipment having more than two wheels can have belt-and-sheave drive systems that drive either a pair of front wheels or a pair of rear wheels. Front wheel drive systems avoid some of the disadvantages of rear wheel drive systems but at the cost of creating other disadvantages. Using a typical four-wheeled, walk-behind lawnmower as an example, cornering effort decreases and wheel slippage is generally avoided because the user can elevate the front drive wheels by pushing downward on a rearwardly-projecting handle when cornering. This vertically pivots the lawnmower about the rear wheels, lifting the front wheels out of contact with the ground below. Because the drive wheels go out of contact with the ground, the drive wheels can rotate freely while the user turns the lawnmower using the independently-rotating rear wheels.
Two disadvantages to front wheel drive systems, however, are that damage to the ground's surface and accelerated tire wear can occur when the drive wheels abruptly re-engage the ground. Another disadvantage is that the front drive wheels can lose traction due to the weight of the equipment shifting as it is pushed up a hill. Because the user's effort greatly increases when pushing a piece of equipment uphill, this is a time that the user particularly needs the drive system to propel the equipment.
Other examples of simple drive systems used on self-propelled, walk-behind equipment are wheel-on-wheel drive systems that drive one or more wheels. Wheel-on-wheel drive systems are similar to belt-and-sheave drive systems. The difference is that the ground-engaging drive wheels are driven by one or more power transfer wheels instead of a belt. When the user engages the drive system, the transfer wheels engage the ground-engaging wheels. If a single transfer wheel is used, a common axle connects the ground-engaging wheels. A wheel-on-wheel drive system, however, has the same disadvantages as the belt-and-sheave drive systems, i.e., increased turning effort, slippage, damage to surface of ground, accelerated tire wear and/or loss of traction.
Complex drive systems for self-propelled, walk-behind power equipment generally provide a differential between the pair of drive wheels. The differential permits independent or differential rotation of the drive wheels on an axle when the user comers. The major disadvantages of complex drive systems are that they are more expensive and heavier than simple systems.